Conventionally, a variable valve timing apparatus is known in the art, which is mounted in an internal combustion engine for a vehicle, and according to which a valve timing (a valve opening and/or valve closing timing) for an intake valve and/or an exhaust valve is changed, in order to increase engine output power, to improve fuel consumption ratio, to decrease emission of harmful components contained in exhaust gas, and so on. In most of the variable valve timing apparatuses, which have been put in a market, a rotational phase (a cam shaft phase) of a cam shaft with respect to a crank shaft is changed by an electric motor or oil pressure (a hydraulic actuator) to thereby change valve timings of the intake valve and the exhaust valve driven by the cam shaft.
According to a variable valve timing apparatus having an electric motor as a driving source, for example, as disclosed in Japanese Patent No. 4,267,635, the cam shaft phase (valve timing) is changed during engine operation is stopped. And an operating amount of the electric motor during an engine stopped period is made smaller than that during an engine operating period, in order to decrease operating sound of the variable valve timing apparatus during the engine stopped period.
When the cam shaft phase is changed during the engine operation is stopped (that is, the rotation of the crank shaft as well as the cam shaft is stopped), the variable valve timing apparatus is operated from its standing-still. Therefore, it is necessary to make an output torque of the electric motor at a larger value, so that the output torque of the electric motor overcomes static friction forces of respective portions of the variable valve timing apparatus to drive the same.
However, the necessary output torque is not considered in the above mentioned prior art (JP Patent No. 4,267,635). According to the above prior art, when the cam shaft phase is changed during the engine operation is stopped, the operating amount of the electric motor is simply made smaller than that during the engine operating period. It may happen that the output torque of the electric motor may come short and thereby the variable valve timing apparatus may not be driven. As a result, it may happen that the cam shaft phase may not be changed.